Exhaust Purification System of Internal Combustion Engine

ABSTRACT

An NOx adsorbent is arranged in an exhaust passage of an internal combustion engine, a fuel addition valve ( 28 ) is arranged in the exhaust passage upstream of the NOx adsorbent, and, when the NOx adsorbent should be made to release the NOx, the fuel addition valve ( 28 ) adds fuel to the NOx adsorbent in the required fuel addition amount to make the air-fuel ratio of the exhaust gas flowing into the NOx adsorbent temporarily rich. In this case, the required fuel addition amount is added divided into a plurality of operations. The fuel addition rate of the fuel addition valve ( 28 ) is detected and the overall addition time from the start of the initial divided addition to the end of the final divided addition is corrected in accordance with the fuel addition rate. Further, the divided addition time, interval, or number of divided additions is corrected so that the amount of fuel actually added from the fuel addition valve ( 28 ) is maintained at the required fuel addition amount.

TECHNICAL FIELD

The present invention relates to an exhaust purification system of aninternal combustion engine.

BACKGROUND ART

Known is an internal combustion engine arranging in an engine exhaustpassage an NOx adsorbent absorbing NOx in exhaust gas when the air-flowratio of the inflowing exhaust gas is lean and releasing the adsorbedNOx when the air-fuel ratio of the inflowing exhaust gas becomes rich,arranging a fuel addition valve in the engine exhaust passage upstreamof the NOx adsorbent, adding fuel to the NOx adsorbent in the requiredfuel addition amount so that the air-fuel ratio of the exhaust gasflowing into the NOx adsorbent becomes temporarily rich when the NOxadsorbent should be made to release NOx, and adding the required fueladdition amount divided into a plurality of operations. In this internalcombustion engine, the NOx generated when fuel is burned under a leanair-fuel ratio is absorbed by the NOx adsorbent. On the other hand, ifthe NOx absorption ability of the NOx adsorbent approaches saturation,the air-fuel ratio of the exhaust gas is temporarily made rich andthereby NOx is released from the NOx adsorbent and reduced.

However, if the abnormality occurs of the port of the fuel additionvalve becoming clogged by so-called deposits, the fuel addition rate ofthe fuel addition valve will fall and the amount of fuel actuallysupplied from the fuel addition valve will be insufficient compared withthe required fuel addition amount, so sufficient release and reductionof NOx will become difficult.

Therefore, there is known an internal combustion engine designed so thatwhen an abnormality occurs in the fuel addition valve, the dividedaddition time is corrected to extend it and the amount of fuel actuallyadded from the fuel addition valve is maintained at the required fueladdition amount (see Japanese Patent Publication (A) No. 2002-242663).

However, if correcting the divided addition time to extend it, theoverall addition time from the start of the initial divided addition tothe end of the final divided addition becomes longer. At this time, theamount of fuel added from the fuel addition valve is maintained at therequired fuel addition amount, so the degree of richness of the air-fuelratio of the exhaust gas flowing in when fuel is added becomes smallerand the NOx is liable to be unable to be sufficiently released andreduced. That is, when correcting the divided injection time to extendit, it is necessary to correct the overall addition time to shorten itso as to reliably release and reduce the NOx.

The above Japanese Patent Publication (A) No. 2002-242663 describes thatwhen correcting the divided injection time to extend it, the overalladdition time not be allowed to become unnecessarily long (see JapanesePatent Publication (A) No. 2002-242663, [0026] etc.), but does notdescribe to correct the overall addition time to shorten it or how tocorrect it to shorten it.

DISCLOSURE OF THE INVENTION

Therefore, an object of the present invention is to provide an exhaustpurification system of an internal combustion engine able to reliablyrelease and reduce NOx even when the fuel addition rate of a fueladdition valve fluctuates from the regular value.

According to the present invention, there is provided an exhaustpurification system of an internal combustion engine provided with anNOx adsorbent arranged in an engine exhaust passage, the NOx adsorbentabsorbing NOx in exhaust gas when the inflowing exhaust gas has a leanair-fuel ratio and releasing the absorbed NOx when the inflowing exhaustgas has a rich air-fuel ratio, a fuel addition valve arranged in theengine exhaust passage upstream of the NOx adsorbent, an additioncontrolling means for adding fuel from the fuel addition valve to theNOx adsorbent in the required fuel addition amount when the NOxadsorbent should be made to release the NOx so that the air-fuel ratioof the exhaust gas flowing into the NOx adsorbent becomes temporarilyrich, the addition controlling means performing divided addition addingthe required fuel addition amount of fuel divided into a plurality ofoperations, a detecting means for detecting a fuel addition rate of thefuel addition valve or the amount of fluctuation of the fuel additionrate with respect to a regular value, and a correcting means forcorrecting the overall addition time from a start of an initial dividedaddition to an end of a final divided addition to shorten it andcorrecting control parameters of divided addition in accordance with thedetected fuel addition rate or amount of fluctuation of the fueladdition rate so that the amount of fuel actually added from the fueladdition valve is maintained at the required fuel addition amount.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an overview of a compression ignition type internal combustionengine, FIG. 2 is a side cross-sectional view of an NOx storingreduction catalyst, FIGS. 3A and 3B are cross-sectional views of asurface part of a catalyst carrier, FIGS. 4A and 4B are views showingthe structure of a particulate filter, FIG. 5 is a time chart forexplaining the NOx release control, FIG. 6 is a view showing a map of anNOx absorption amount dNOx per unit time, FIG. 7 is a view showing a mapof the required fuel addition amount Q, FIG. 8 is a time chart forexplaining a fuel addition parameter, FIG. 9 is a view showing therelationship between an overall addition time tALL and NOx purificationrate EFF, FIG. 10 is a view showing the relationship of the overalladdition time tALL and NOx purification rate EFF, FIG. 11 is a viewshowing a map of an overall addition time optimum value tAM, FIG. 12 isa view showing a map of an overall addition time optimum value tAM,FIGS. 13A, 13B, and 13C are time charts for explaining fuel additionwhen correcting the overall addition time tALL to shorten it, FIG. 14 isa flow chart for NOx release control, FIG. 15 is a view showing anotherembodiment of a compression ignition type internal combustion engine,FIG. 16 is a time chart for explaining another embodiment according tothe present invention, FIG. 17 is a flow chart for NOx release controlof another embodiment according to the present invention, FIG. 18 is aview showing still another embodiment according to the presentinvention, FIG. 19 is a view showing the relationship between theoverall addition time tALL and exhausted HC amount, FIG. 20 is a timechart for explaining still another embodiment according to the presentinvention, and FIG. 21 is a flow chart for NOx release control of stillanother embodiment according to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 shows the case of application of the present invention to acompression ignition type internal combustion engine. However, thepresent invention can also be applied to a spark ignition type internalcombustion engine.

Referring to FIG. 1, 1 shows an engine body, 2 a combustion chamber ofeach cylinder, 3 an electronic control type fuel injector for injectingfuel into each combustion chamber 2, 4 an intake manifold, and 5 anexhaust manifold. The intake manifold 4 is connected through an intakeduct 6 to an outlet of a compressor 7 a of an exhaust turbocharger 7,while an inlet of the compressor 7 a is connected through an air flowmeter 8 to an air cleaner 9. Inside the intake duct 6 is arranged athrottle valve 10. Further, around the intake duct 6 is arranged acooling device 11 for cooling the intake air flowing inside the intakeduct 6. In the embodiment shown in FIG. 1, the engine cooling water isguided into the cooling device 11 where the engine cooling water is usedto cool the intake air. On the other hand, the exhaust manifold 5 isconnected to the inlet of the exhaust turbine 7 b of the exhaustturbocharger 7, while the outlet of the exhaust turbine 7 b is connectedto an exhaust post-treatment device 20.

The exhaust manifold 5 and the intake manifold 4 are connected with eachother through an exhaust gas recirculation (hereinafter referred to asan “EGR”) passage 12. Inside the EGR passage 12, an electrical controltype EGR control valve 13 is arranged. Further, around the EGR passage12, a cooling device 14 is arranged for cooling the EGR gas flowingthrough the inside of the EGR passage 12. In the embodiment shown inFIG. 1, engine cooling water is guided into the cooling device 14 wherethe engine cooling water is used to cool the EGR gas. On the other hand,the fuel injectors 3 are connected through fuel feed pipes 15 to acommon rail 16. This common rail 16 is supplied with fuel from anelectronic control type variable discharge fuel pump 17. The fuelsupplied into the common rail 16 is supplied through the fuel feed pipes15 to the fuel injectors 3.

The exhaust post-treatment device 20 is provided with an exhaust pipe 21connected to an outlet of an exhaust turbine 7 b, a catalytic converter22 connected to the exhaust pipe 21, and an exhaust pipe 23 connected tothe catalytic converter 22. Inside the catalytic converter 22 arearranged, in order from the upstream side, an NOx storing reductioncatalyst 24 and particulate filter 25. Further, the exhaust pipe 23 isprovided with a temperature sensor 26 for detecting the temperature ofthe exhaust gas exhausted from the catalytic converter 22 and anair-fuel ratio sensor 27 for detecting the air-fuel ratio of the exhaustgas exhausted from the catalytic converter 22. The temperature of theexhaust gas exhausted from the catalytic converter 22 expresses thetemperature of the NOx storing reduction catalyst 24 and particulatefilter 25.

On the other hand, as shown in FIG. 1, the exhaust manifold 5 has a fueladdition valve 28 attached to it. This fuel addition valve 28 issupplied with fuel from the common rail 16. Fuel is added from the fueladdition valve 28 to the inside of the exhaust manifold 5. In thisembodiment according to the present invention, this fuel is comprised ofdiesel oil. Note that the fuel addition valve 28 can also be attached tothe exhaust pipe 21.

The electronic control unit 30 is comprised of a digital computer and isprovided with a ROM (read only memory) 32, RAM (random access memory)33, CPU (microprocessor) 34, input port 35, and output port 36 connectedwith each other by a bi-directional bus 31. The output signals of theair flow meter 8, temperature sensor 26, and air-fuel ratio sensor 27are input through the corresponding AD converters 37 to the input port35. Further, the accelerator pedal 39 has connected to it a load sensor50 generating an output voltage proportional to the amount of depressionL of the accelerator pedal 39. The output voltage of the load sensor 40is input through the corresponding AD converter 37 to the input port 35.Further, the input port 35 has a crank angle sensor 41 connected to itgenerating an output pulse each time the crankshaft rotates by forexample 15°. The CPU 34 calculates the engine speed N based on theoutput pulse of the crank angle sensor 41. On the other hand, the outputport 36 is connected through the corresponding drive circuits 38 to thefuel injectors 3, throttle valve 10 drive device, EGR control valve 13,fuel pump 17, and fuel addition valve 28.

FIG. 2 shows the structure of the NOx storing reduction catalyst 24. Inthe embodiment shown in FIG. 2, the NOx storing reduction catalyst 24forms a honeycomb structure and is provided with a plurality of exhaustgas flow passages 61 separated from each other by thin partition walls60. On the two side surfaces of each partition wall 60, for example, acatalyst carrier comprised of alumina is carried. FIGS. 3A and 3Bschematically show cross-sections of the surface parts of this catalystcarrier 65. As shown in FIGS. 3A and 3B, the catalyst carrier 65 has aprecious metal catalyst 66 carried dispersed on its surface. Further,the catalyst carrier 65 has a layer of an NOx adsorbent 67 formed on itssurface.

In the embodiment according to the present invention, as the preciousmetal catalyst 66, platinum Pt is used. As the ingredient forming theNO_(x) adsorbent 67, for example, at least one ingredient selected frompotassium K, sodium Na, cesium Cs, or another such alkali metal, bariumBa, calcium Ca, or another such alkali earth, and lanthanum La, yttriumY, or another such rare earth is used.

If the ratio of the air and fuel (hydrocarbons) supplied inside theengine intake passage, combustion chambers 2, and exhaust passageupstream of the NOx storing reduction catalyst 24 is referred to as the“air-fuel ratio of the exhaust gas”, the NO_(x) adsorbent 67 absorbs theNO_(x) when the air-fuel ratio of the exhaust gas is lean and releasesthe absorbed NO_(x) when the oxygen concentration in the exhaust gasfalls—in an “NO_(x) absorption/release action”.

That is, explaining the case of using barium Ba as the ingredientforming the NO_(x) adsorbent 67 as an example, when the air-fuel ratioof the exhaust gas is lean, that is, when the oxygen concentration inthe exhaust gas is high, the NO contained in the exhaust gas, as shownin FIG. 3A, is oxidized on the platinum Pt 66 and becomes NO₂, next thisis absorbed in the NO_(x) adsorbent 67 and, while bonding with thebarium oxide BaO, diffuses in the form of nitric acid ions NO₃ ⁻ insidethe NO_(x) adsorbent 67. In this way, the NO_(x) is absorbed inside theNO_(x) adsorbent 67. So long as the oxygen concentration in the exhaustgas is high, NO₂ is produced on the surface of the platinum Pt 66. Solong as the NO_(x) adsorption ability of the NO_(x) adsorbent 67 is notsaturated, the NO₂ is absorbed in the NO_(x) adsorbent 67 and nitricacid ions NO₃ ⁻ are produced.

As opposed to this, if the air-fuel ratio of the exhaust gas is maderich or the stoichiometric air-fuel ratio, the oxygen concentration inthe exhaust gas falls, so the reaction proceeds in the oppositedirection (NO₃ ⁻→NO₂) and therefore, as shown in FIG. 3B, the nitricacid ions NO₃ ⁻ in the NO_(x) adsorbent 67 are released in the form ofNO₂ from the NO_(x) adsorbent 67. Next, the released NO_(x) is reducedby the unburned HC and CO contained in the exhaust gas.

In this way, when the air-fuel ratio of the exhaust gas is lean, thatis, when combustion is performed under a lean air-fuel ratio, the NO_(x)in the exhaust gas is absorbed in the NO_(x) adsorbent 67. However, whencombustion continues under a lean air-fuel ratio, during that time theNO_(x) adsorption ability of the NO_(x) adsorbent 67 ends up becomingsaturated and therefore the NO_(x) adsorbent 67 ends up no longer beingable to absorb the NO_(x). Therefore, in the embodiment according to thepresent invention, before the adsorption ability of the NO_(x) adsorbent67 becomes saturated, fuel is supplied from the fuel addition valve 28so as to temporarily make the air-fuel ratio of the exhaust gas rich andthereby make the NO_(x) be released from the NO_(x) adsorbent 67.

FIGS. 4A and 4B show the structure of the particulate filter 25. Notethat FIG. 4A shows a front view of the particulate filter 25, while FIG.4B shows a side cross-sectional view of the particulate filter 25. Asshown in FIGS. 4A and 4B, the particulate filter 25 forms a honeycombstructure and is provided with a plurality of exhaust flow passages 70,71 extending in parallel with each other. These exhaust flow passagesare comprised of exhaust gas inflow passages 70 with downstream endsclosed by plugs 72 and exhaust gas outflow passages 71 with upstreamends closed by plugs 73. Note that the hatched parts in FIG. 4A show theplugs 73. Therefore, the exhaust gas inflow passages 70 and exhaust gasoutflow passages 71 are alternately arranged via thin partition walls74. In other words, the exhaust gas inflow passages 70 and exhaust gasoutflow passages 71 are arranged so that each exhaust gas inflow passage70 is surrounded by four exhaust gas outflow passages 71 and eachexhaust gas outflow passage 71 is surrounded by four exhaust gas inflowpassages 70.

The particulate filter 25 is for example formed from a porous materialsuch as cordierite. Therefore, the exhaust gas flowing into the exhaustgas inflow passage 70, as shown by the arrows in FIG. 4B, passes throughthe surrounding partition walls 74 and flows out into the adjoiningexhaust gas outflow passages 71.

In the embodiment according to the present invention, the peripheralwalls of the exhaust gas inflow passages 70 and exhaust gas outflowpassages 71, that is, the two side surfaces of the partition walls 74and the inside walls of the fine holes in the partition walls 74, carry,for example, a catalyst carrier comprised of alumina. On the surface ofthe catalyst carrier 65, as shown in FIGS. 3A and 3B, a precious metalcatalyst 66 comprised of platinum Pt is carried diffused in it and alayer of an NO_(x) adsorbent 67 is formed.

Therefore, when the fuel is burned under a lean air-fuel ratio, the NOxin the exhaust gas is also absorbed in the NOx adsorbent 67 on theparticulate filter 25. The NOx absorbed in this NOx adsorbent 67 isreleased by the fuel addition valve 28 adding fuel.

On the other hand, the particulate matter contained in the exhaust gasis trapped on the particulate filter 25 and is successively oxidized.However, if the trapped amount of particulate matter becomes greaterthan the amount of oxidized particulate matter, the particulate matteris gradually deposited on the particulate filter 25. In this case, ifthe deposited amount of the particulate matter increases, a drop in theengine output ends up being invited. Therefore, when the depositedamount of the particulate matter increases, the deposited particulatematter must be removed. In this case, if raising the temperature of theparticulate filter 25 under an excess of air to about 600° C., thedeposited particulate matter will be oxidized and removed.

Therefore, in the embodiment according to the present invention, whenthe amount of particulate matter deposited on the particulate filter 25exceeds the allowable amount, while maintaining the air-fuel ratio ofthe exhaust gas flowing into the particulate filter 25 lean, the fueladdition valve 28 adds fuel, the heat of oxidation reaction of the addedfuel is used to raise the temperature of the particulate filter 25, andthereby the deposited particulate matter is removed by oxidation.

Note that in FIG. 1, the NOx storing reduction catalyst 24 can beomitted. Further, as the particulate filter 25 in FIG. 1, a particulatefilter not carrying an NOx adsorbent 67 can be used.

Now, in the embodiment according to the present invention, as shown by Xin FIG. 5, each time the NOx amount cumulative value ΣNOx exceeds theallowable value MAX, fuel is added from the fuel addition valve 28 andthe air-fuel ratio of the exhaust gas flowing into the NOx adsorbent 67carried on the NOx storing reduction catalyst 24 and on the particulatefilter 25 is temporarily switched to rich. As a result, NOx is releasedfrom the NOx adsorbent 67 and reduced.

In this case, in the embodiment according to the present invention, theNOx amount dNOx absorbed in the NOx adsorbent 67 per unit time is storedas a function of the required torque TQ and engine speed N in the formof a map as shown in FIG. 6 in advance in the ROM 32. By cumulativelyadding this NOx amount dNOx, the cumulative value ΣNOx of the NOx amountabsorbed in the NOx adsorbent 67 is calculated.

On the other hand, for switching the air-fuel ratio of the exhaust gasflowing into the NOx adsorbent 67 to rich, the fuel is added from thefuel addition valve 28 by the required fuel addition amount Q. Thisrequired fuel addition amount Q is for example stored as a function ofthe intake air amount Ga and temperature Tc of the NOx adsorbent 67 inthe form of a map as shown in FIG. 7 in advance in the ROM 32.

As will be understood from FIG. 5, in the embodiment according to thepresent invention, the required fuel addition amount Q is added dividedinto a plurality of operations. In the example shown in FIG. 8, thedivided addition of the time tDIV is performed four times separated bythe intervals tINT. Here, if the number of divided additions isexpressed by n (=2,3, . . . ), the overall addition time tALL from thestart of the initial divided addition to the end of the final dividedaddition is expressed by the following formula:

tALL=n·tDIV+(n−1)·tINT

In this case, the fact that if the overall addition time tALL changes,even if maintaining the fuel addition amount constant, the NOxpurification rate EFF of the NOx adsorbent 67 changes was confirmed bythe present inventors. This will be explained while referring to FIG. 9.Here, if the amount of NOx flowing into the NOx adsorbent 67 in the timefrom the end of the previous addition of fuel for release and reductionof NOx to the end of the next addition of fuel for release and reductionof NOx is expressed as “Nin” and the amount of NOx flowing out from theNOx adsorbent 67 as “Nout”, the NOx purification rate EFF can forexample be expressed by (Nin−Nout)/Nin.

FIG. 9 shows the NOx purification rate EFF of the NOx adsorbent 67 whenholding the fuel addition amount constant and changing the overalladdition time tALL. Note that for example by maintaining the dividedaddition time tDIV and number of divisions n and changing the intervaltINT, it is possible to maintain the fuel addition amount constant andchange the overall addition time tALL.

As shown in FIG. 9, there is an optimum value tAM maximizing the NOxpurification rate EFF in the overall addition time tALL. The NOxpurification rate EFF falls as the overall addition time tALL becomesshorter than or longer than the optimum value tAM. This is considered tobe because if the overall addition time tALL becomes shorter, the timeduring which the air-fuel ratio of the exhaust gas flowing into the NOxadsorbent 67 is held rich becomes shorter and if the overall additiontime tALL becomes longer, the degree of richness of the air-fuel ratioof the exhaust gas flowing into the NOx adsorbent 67 becomes smaller.

However, as explained at the start, if the port of the fuel additionvalve 28 is clogged by deposits mainly comprised of solid carbon, thefuel addition amount per unit time of the fuel addition valve 28, thatis, the fuel addition rate q, falls from the value when the fueladdition valve 28 is not clogged, that is, the regular value qp. In thiscase, if correcting the divided addition time tDIV to extend it, theamount of fuel actually added from the fuel addition valve 28 can bemaintained at the required fuel addition amount Q. On top of this, ifcorrecting the interval tINT to shorten it to correct the overalladdition time tALL to shorten it to the optimum value tAMp of when thefuel addition rate q is the regular value qp, it appears that the NOxpurification rate EFF can be maintained at the maximum.

However, the fact that when the fuel addition rate q falls from theregular value qp, even if correcting the overall addition time tALL toshorten it to the above-mentioned tAMp, the NOx purification rate EFFcannot be maintained at the maximum and the overall addition time tALLhas to be further corrected to shorten it was discovered by the presentinventors. This will be explained with reference to FIG. 10.

In FIG. 10, the curve P shows the NOx purification rate EFF when thefuel addition rate q is the regular value qp, while the curve C showsthe NOx purification rate EFF when the fuel addition rate q falls fromthe regular value qp. Note that in each case, the amount of fuelactually added from the fuel addition valve 28 is maintained at therequired fuel addition amount Q.

As will be understood from FIG. 10, when the fuel addition rate q fallsfrom the regular value qp, the NOx purification rate EFF shown by thecurve C shifts in the direction where the overall addition time tALLbecomes shorter and the optimum value tAMc of the overall addition timetALL for maximizing the NOx purification rate EFF becomes shorter thanthe optimum value tAMp when the fuel addition rate q is the regularvalue qp. This is believed to be because of the following reasons. Thatis, if the fuel addition rate q falls, the penetrating force of theadded fuel falls, so the added fuel depositing on the inside walls ofthe exhaust passage increases. The added fuel deposited once on theinside wall surfaces of the exhaust passage later evaporates, separatesfrom the inside wall surfaces of the exhaust passage, and then reachesthe NOx adsorbent 67. For this reason, the added fuel graduallyseparates from the wall inside the exhaust passage and reaches the NOxadsorbent 67. As a result, the degree of richness of the air-fuel ratioof the exhaust gas flowing into the NOx adsorbent 67 becomes smaller.

This being so, to make the degree of richness sufficiently large, it isnecessary to add the fuel in the required fuel addition amount Q in ashort time, that is, it is necessary to further correct the overalladdition time tALL to shorten it.

In this case, the optimum value tAM of the overall addition time tALLwhen the fuel addition rate q falls from the regular value qp, as shownin FIG. 11, becomes shorter the larger the drop Δq (=qp−q) of the fueladdition rate q from the regular value qp or, as shown in FIG. 12,becomes shorter the smaller the fuel addition rate q itself.

Therefore, in the embodiment according to the present invention, thefuel addition rate q or its drop Δq is detected, the optimum value tAMof the overall addition time is calculated in accordance with this fueladdition rate q or its drop Δq, and the overall addition time tALL isset to this optimum value tAM. On top of this, the divided addition timetDIV, interval tINT, or number of divisions n is corrected in accordancewith the detected fuel addition rate q or its drop Δq so that the amountof fuel actually added in this overall addition time tALL matches withthe required fuel addition amount Q.

For example, when the fuel addition rate q is the regular value qp, theaddition time required for adding the required fuel addition amount Q issubstantially (Q/qp), so the divided addition time tDIVp when the fueladdition rate q is the regular value qp can be found from the followingformula:

tDIVp=(Q/qp)/n

This being so, the divided addition time tDIV required for adding therequired fuel addition amount Q at the time of the fuel addition rate qcan be found from the following formula:

tDIV=tDIVp·(qp/q)

Therefore, the interval tINT required for performing the dividedaddition of the divided addition time tDIV n number of times in theoverall addition time tAL can be found from the following formula L:

tINT=(tALL−n·tDIV)/(n−1)

Various methods are known for detecting the fuel addition rate q or itsdrop Δq. For example, it is possible to detect the fuel addition rate qor its drop Δq in accordance with the extent of rise of the NOxadsorbent temperature Tc occurring when actually adding fuel from thefuel addition valve 28. That is, it is learned that at the time of fueladdition, when the rise in the NOx adsorbent temperature Tc detected bythe temperature sensor 26 (FIG. 1) is large, the fuel addition rate q islarge, while when the rise in temperature is small, the fuel additionrate q is small.

In the embodiment according to the present invention, when the fueladdition valve 28 adds fuel so as to release NOx from the NOx adsorbent67 and reduce it, the fuel addition rate q or its addition amount Δq isdetected based on the NOx adsorbent temperature Tc. Based on this fueladdition rate q or its addition amount Δq, the optimum value tAM of theoverall addition time tALL is calculated from the map of FIG. 11 or FIG.12. At the fuel addition for release and reduction of NOx performednext, the overall addition time tALL is set to this optimum value tAM,the required fuel addition amount Q is calculated, and the dividedaddition time tDIV and interval tINT for making the amount of fuelactually added during the overall addition time tALL match with therequired fuel addition amount Q are calculated using the fuel additionrate q or its drop Δq.

FIG. 13A and 13B show an example of addition of fuel when the fueladdition rate q falls below the regular value qp. In the example shownin FIG. 13A, compared with the case where the fuel addition rate q shownin FIG. 13C is the regular value qp, the overall addition time tALL iscorrected to shorten it, the divided addition time tDIV is corrected toextend it, the interval tINT is corrected to shorten it, and the numberof divided additions n is maintained. On the other hand, in the exampleshown in FIG. 13B, compared with the case shown in FIG. 13C, the overalladdition time tALL is corrected to shorten it, the divided addition timetDIV is corrected to extend it, the number of divided additions n isreduced, and the interval tINT is also corrected to shorten it.

FIG. 14 shows the NOx release control routine.

Referring to FIG. 14, first, at step 100, the NOx amount ΣNOx absorbedin the NOx adsorbent 67 is calculated. In the embodiment according tothe present invention, the NOx amount dNOx absorbed per unit time iscalculated from the map shown in FIG. 6, and this dNOx is added to theNOx amount ΣNOx absorbed in the NOx adsorbent 67. Next, at step 101, itis determined if the absorbed NOx amount ΣNOX has exceeded the allowablevalue MAX. When ΣNOX>MAX, the routine proceeds to step 102 where therequired fuel addition amount Q is calculated from the map of FIG. 7. Atthe next step 103, the fuel addition parameter is calculated. That is,the divided addition time tDIV, interval tINT, and number of dividedadditions n required for addition of the required fuel addition amount Qcalculated at step 102 in the overall addition time tALL preset at step107 of the previous processing cycle are calculated using the fueladdition rate q or its drop Δq detected at step 106 of the previousprocessing cycle. At the next step 104, fuel is added based on the fueladdition parameter determined at step 103. At the next step 105, theabsorbed NOx amount ΣNOx is returned to zero. At the next step 106, thefuel addition rate q or its drop Δq is detected. At the next step 107,the overall addition time optimum value tAM is calculated from the mapof FIG. 11 or FIG. 12 based on the fuel addition rate q or its drop Δqdetected at step 106. This optimum value tAM is set as the overalladdition time tALL for the next fuel addition.

Note that it is also possible that when the detected fuel addition rateq is larger than a threshold value or its drop Δq is smaller than athreshold value, the overall addition time tALL, divided addition timetDIV, interval tINT, or number of divided additions n are not correctedand when the fuel addition rate q becomes smaller than a threshold valueor its drop Δq becomes larger than a threshold value, the overalladdition time tALL etc. are corrected.

FIG. 15 shows another embodiment according to the present invention. Theembodiment shown in FIG. 15 differs in configuration from the embodimentof FIG. 1 in the point that an NOx sensor 80 is attached to the exhaustpipe 23 for detecting the NOx amount or NOx concentration flowing outfrom the NOx adsorbent 67 and the point that an alarm device 81 isprovided for indicating a breakdown of the fuel addition valve 28. Theoutput signal of the NOx sensor 80 is input through the corresponding ADconverter 37 to the input port 35, the alarm device 81 is connectedthrough the corresponding drive device 38 to the output port 36, andcontrol is performed based on the output signal from the electroniccontrol unit 30.

As explained above, when the fuel addition rate q of the fuel additionvalve 28 falls, if the overall addition time tALL is held constant, theNOx purification rate of the NOx adsorbent 67 falls. Therefore, inanother embodiment according to the present invention, the NOxpurification rate EFF of the NOx adsorbent 67 is detected based on theoutput of the NOx sensor 80. The overall addition time tALL is correctedso that this NOx purification rate EFF is maintained at the maximum.

That is, deposits clog the fuel addition valve 28 along with the elapseof time, therefore fuel addition rate q becomes smaller along with theelapse of time and the fuel addition rate drop Δq becomes greater alongwith the elapse of time. On the other hand, as explained referring toFIG. 9 or FIG. 10, when the fuel addition rate q falls, if correctingthe overall addition time tALL to shorten it, the NOx purification rateEFF can be increased. Therefore, in another embodiment according to thepresent invention, each time fuel is added to release and reduce NOx,the overall addition time tALL is shortened.

However, as will be understood from FIG. 9, if the overall addition timetALL becomes shorter than the optimum value tAM, the shorter the overalladdition time tALL becomes, the more the NOx purification rate EFFdrops. Therefore, in another embodiment according to the presentinvention, when as a result of the correction of the overall additiontime tALL to shorten it, the overall addition time tALL is corrected toshorten it so long as the NOx purification rate EFF rises. When as aresult of the correction of the overall addition time tALL to shortenit, the NOx purification rate EFF falls, the overall addition time tALLis corrected to extend it. In this way, the overall addition time tALLcan be maintained at the optimum value tAM and the NOx purification rateEFF can be maintained at the maximum.

In FIG. 16, X indicates the timing when fuel is added for release andreduction of NOx. As shown in FIG. 16, the overall addition time tALL isfor example corrected to shorten it by a small predetermined value ΔY.As a result, when the NOx purification rate EFF rises, the overalladdition time tALL is further corrected to shorten it by ΔY. As opposedto this, when as a result of the overall addition time tALL beingcorrected to shorten it, the NOx purification rate EFF falls, theoverall addition time tALL is extended by ΔY, that is, is returned tothe original value. In this way, the NOx purification rate EFF ismaintained at the maximum.

For example, when the clogging of the fuel addition valve 28 becomesserious and the fuel addition rate q becomes considerably small, even ifcorrecting the overall addition time tALL to shorten it, the NOxpurification rate EFF can no longer be maintained at the maximum and isliable to fall below the previous NOx purification rate EFF0. Therefore,when the drop (=EFF0−EFF) of the NOx purification rate EFF as a resultof correction of the overall addition time tALL to shorten it from theprevious NOx purification rate EFF0 is larger than the allowable value,it is determined that the fuel addition valve 28 has broken and thealarm device 81 is actuated. Note that when the NOx purification rateEFF is lower than the allowable limit as a result of correction of theoverall addition time tALL to shorten it, it can be determined that thefuel addition valve 28 is broken.

FIG. 17 shows the NOx release control routine of another embodimentaccording to the present invention.

Referring to FIG. 17, first, at step 200, the NOx amount ΣNOx absorbedin the NOx adsorbent 67 is calculated. Next, at step 201, it isdetermined whether the absorbed NOx amount ΣNOX has exceeded theallowable value MAX. When ΣNOX>MAX, the routine proceeds to step 202where the required fuel addition amount Q is calculated from the map ofFIG. 7. At the next step 203, the fuel addition parameter is calculated.That is, the divided addition time tDIV, interval tINT, and number ofdivided additions n required for addition of the required fuel additionamount calculated at step 202 in the overall addition time tALL presetat step 209 or step 212 of the previous processing cycle are calculatedusing the fuel addition rate q or its drop Δq detected at step 206 ofthe previous processing cycle. At the next step 204, the fuel is addedbased on the fuel addition parameter determined at step 203. At the nextstep 205, the absorbed NOx amount ΣNOx is returned to zero. At the nextstep 206, the fuel addition rate q or its drop Δq is detected. At thenext step 207, the NOx purification rate EFF of the NOx adsorbent 67 isdetected. At the next step 208, it is determined if the current NOxpurification rate EFF detected at step 207 is higher than the previousNOx purification rate EFF0. When EFF>EFF0, next, the routine proceeds tostep 209, where the overall addition time tALL is shortened by thepredetermined value AY. Next, the routine proceeds to step 210, wherethe current NOx purification rate EFF is made EFF0. As opposed to this,when EFF<EFF0, the routine proceeds from step 208 to step 211, where itis determined if the drop (=EFF0−EFF) of the current NOx purificationrate EFF from the previous NOx purification rate EFF0 is larger than anallowable value LMT. When EFF00−EFF<LMT, next, the routine proceeds tostep 212, where the overall addition time tALL is extended by apredetermined value ΔY. Next, the routine proceeds to step 210. Asopposed to this, when EFF0−EFF>LMT, next, the routine proceeds to step213, where the alarm device 82 is actuated.

FIG. 18 shows still another embodiment according to the presentinvention. The embodiment shown in FIG. 18 differs in configuration fromthe embodiment of FIG. 15 in the point of an HC sensor 82 for detectingthe amount of HC or HC concentration in the exhaust gas flowing out fromthe NOx adsorbent 67 being attached to the exhaust pipe 23. The outputsignal of the HC sensor 82 is input through the corresponding ADconverter 37 to the input port 35.

FIG. 19 shows the HC amount QHC exhausted from the NOx adsorbent 67 whenfuel is added when holding the fuel addition amount constant andchanging the overall addition time tALL. In FIG. 19, the curve P showsthe HC amount QHC exhausted when the fuel addition rate q is the regularvalue qp, while curve C shows the HC amount QHC exhausted when the fueladdition rate q falls from the regular value qp.

Looking at for example the curve P of FIG. 19, as the overall additiontime tALL becomes shorter, the HC amount QHC exhausted becomes greater,while as the overall addition time tALL becomes longer, the HC amountQHC exhausted becomes smaller. On the other hand, if the fuel additionrate q falls from the regular value qp, the NOx purification rate EFFshown by the curve C shifts in a direction where the overall additiontime tALL becomes shorter. This is because, as explained above, if thefuel addition rate q falls, the degree of richness of the air-fuel ratioof the exhaust gas flowing into the NOx adsorbent 67 becomes smaller.

This being so, it is learned that when the HC amount QHC exhausted whenadding fuel is small, the degree of richness of the inflowing exhaustgas becomes smaller and the fuel addition rate q falls.

Therefore, in still another embodiment according to the presentinvention, the HC amount QHC exhausted when fuel is added is detected bythe HC sensor 82 and the overall addition time tALL is corrected so thatthis exhausted HC amount QHC matches with the target value QHCt. Thatis, in the example shown in FIG. 19, the overall addition time tALL iscorrected by being shortened from tAMpp to tAMcc. As a result, a largeamount of HC being exhausted from the NOx adsorbent 67 can be prevented,the degree or richness of the air-fuel ratio of the inflowing exhaustgas can be maintained high, and the NOx can be released and reducedwell.

In FIG. 20, X shows the timing for addition of fuel for release andreduction of NOx. As shown in FIG. 20, when the exhausted HC amount QHCis smaller than the target value QHCt, the overall addition time tALL iscorrected to be shortened by for example a small predetermined value AZ.As opposed to this, if the overall addition time tALL becomes largerthan the target value QHCt, the overall addition time tALL is extendedby ΔZ, that is, is returned to the original value. In this way, theexhausted HC amount QHC is maintained at the target value QHCt.

In this case, if setting the exhausted HC amount for maximizing the NOxpurification rate EFF at the target value QHCt, it is possible tomaintain the NOx purification rate EFF at the maximum.

FIG. 21 shows the NOx release control routine of still anotherembodiment according to the present invention.

If referring to FIG. 21, first, at step 300, the NOx amount ΣNOxabsorbed in the NOx adsorbent 67 is calculated. Next, at step 301, it isdetermined if the absorbed NOx amount ΣNOX has exceeded an allowablevalue MAX. When ΣNOX>MAX, the routine proceeds to step 302 where therequired fuel addition amount Q is calculated from the map of FIG. 7. Atthe next step 303, the fuel addition parameter is calculated. That is,the divided addition time tDIV, interval tINT, and number of dividedadditions n required for addition of the required fuel addition amountcalculated at step 302 in the overall addition time tALL preset at step309 or step 310 of the previous processing cycle are calculated usingthe fuel addition rate q or its drop Δq detected at step 306 of theprevious processing cycle. At the next step 304, fuel is added based onthe fuel addition parameter determined at step 303. At the next step305, the absorbed NOx amount ΣNOx is returned to zero. At the next step306, the fuel addition rate q or its drop Δq is detected. At the nextstep 307, the HC amount QHC exhausted from the NOx adsorbent 67 isdetected. At the next step 308, it is determined whether the exhaustedHC amount QHC detected at step 307 is smaller than the target valueQHCt. When QHC<QHCt, next, the routine proceeds to step 309, where theoverall addition time tALL is shortened by the predetermined value ΔZ.As opposed to this, when QHC≧QHCt, the routine proceeds from step 308 tostep 310, where the overall addition time tALL is extended by apredetermined value ΔZ.

Note that when the HC amount QHC exhausted when correcting the overalladdition time tALL to shorten it falls compared with the HC amount QHC0exhausted at the time of the previous fuel addition and the drop at thistime (=QHC0−QHC) becomes larger than an allowable value, it may bedetermined that the fuel addition valve 28 has broken down and an alarmdevice 81 may be actuated. Alternatively, when the HC amount QHCexhausted when correcting the overall addition time tALL to shorten itis smaller than the allowable limit, it may be determined that the fueladdition valve 28 has broken down.

1. An exhaust purification system of an internal combustion engineprovided with: an NOx adsorbent arranged in an engine exhaust passage,said NOx adsorbent absorbing NOx in exhaust gas when the inflowingexhaust gas has a lean air-fuel ratio and releasing the absorbed NOxwhen the inflowing exhaust gas has a rich air-fuel ratio, a fueladdition valve arranged in the engine exhaust passage upstream of saidNOx adsorbent, an addition controlling means for adding fuel from thefuel addition valve to the NOx adsorbent in the required fuel additionamount when the NOx adsorbent should be made to release the NOx so thatthe air-fuel ratio of the exhaust gas flowing into the NOx adsorbentbecomes temporarily rich, said addition controlling means performingdivided addition adding said required fuel addition amount of fueldivided into a plurality of operations, a detecting means for detectinga fuel addition rate of the fuel addition valve or the amount offluctuation of the fuel addition rate with respect to a regular value,and a correcting means for correcting the overall addition time from astart of an initial divided addition to an end of a final dividedaddition to shorten it and correcting control parameters of dividedaddition in accordance with said detected fuel addition rate or amountof fluctuation of the fuel addition rate so that the amount of fuelactually added from the fuel addition valve is maintained at therequired fuel addition amount.
 2. An exhaust purification system of aninternal combustion engine as set forth in claim 1, wherein said controlparameters of divided addition are a divided addition time and aninterval from a previous divided addition to a next divided addition ornumber of divided additions.
 3. An exhaust purification system of aninternal combustion engine as set forth in claim 1, wherein saidcorrecting means corrects the overall addition time so as to becomeshorter the smaller said fuel addition rate or the larger the drop ofsaid fuel addition rate.
 4. An exhaust purification system of aninternal combustion engine as set forth in claim 1, wherein the systemis further provided with an NOx purification rate detecting means fordetecting an NOx purification rate of the NOx adsorbent and saidcorrecting means corrects the overall addition time to shorten it orcorrects it to extend it so that said detected NOx purification ratematches a target value.
 5. An exhaust purification system of an internalcombustion engine as set forth in claim 1, wherein the system is furtherprovided with an HC amount detecting means for detecting an amount of HCin the exhaust gas flowing out from the NOx adsorbent when fuel is addedand said correcting means corrects the overall addition time to shortenit or corrects it to extend it so that said detected HC amount matches atarget amount.
 6. An exhaust purification system of an internalcombustion engine as set forth in claim 1, wherein the system is furtherprovided with an NOx purification rate detecting means for detecting anNOx purification rate of the NOx adsorbent and a determining means fordetermining that the fuel addition valve is broken when the drop in thedetected NOx purification rate is larger than an allowable value or saiddetected NOx purification rate is lower than an allowable limit even ifcorrecting the overall addition time to shorten it and correct thecontrol parameters of the divided addition.